The peak accelerations recorded on alluvial sites during the Northridge earthquake
were about 50% larger than the median value predicted by current empirical attenuation
relations at distances less than about 30 km. This raises the question of whether
the ground motions from the Northridge earthquake are anomalous for thrust events,
or are representative of ground motions expected in future thrust earthquakes. Since
the empirical data base contains few strong motion records close to large thrust
earthquakes, it is difficult to assess whether the Northridge ground motions are
anomalous based on recorded data alone. We have used a broadband strong motion
simulation procedure to help assess whether the ground motions were anomalous. The
ground motions from the Northridge earthquake and our simulations of these ground
motions have a similar pattern of departure from empirical attenuation relations for
thrust earthquakes: the peak accelerations are at about the 84th percentile level for
distances within 20 to 30 km, and follow the median level for larger distances. This
same pattern of departure from empirical attenuation relations was obtained in our
simulations of the peak accelerations of an Elysian Park blind thrust event prior
to the occurrence of the Northridge earthquake, and from twenty randomly generated
rupture models of future Northridge earthquakes. Since we are able to model this
pattern with broadband simulations, and had done so before the Northridge earthquake
occurred, this suggests that the Northridge strong motion records are not anomalous,
and are representative of ground motions close to thrust faults. Accordingly, it seems
appropriate to include these recordings in strong motion data sets that are used to
develop empirical ground motion attenuation relations for thrust faults, and to use this
augmented data set as the basis for evaluating the need for modifications in design
coefficients in the seismic provisions of building codes.

We evaluated systematic differences in ground motion on the hanging wall and foot wall
during the Northridge earthquake using empirical data. An empirical model for the hanging
wall effect was developed for the Northridge earthquake. This empirical model results
in up to a 50% increase in peak horizontal accelerations on the hanging wall over the
distance range of 10 to 20 km relative to the median attenuation for the Northridge
earthquake. In contrast, the peak accelerations on the foot wall are not significantly
different from the median attenuation over this distance range. Recordings from other
reverse events show a similar trend of an increase in the peak accelerations on the
hanging wall, indicating this systematic difference in hanging wall peak accelerations
is likely to be observed in future reverse events. A modification to the near source
factor in the proposed 1997 revisions to the Uniform Building Code is proposed to accommodate
hanging wall effects near crustal thrust faults in California.